44 PLANT GROWTH 



for that reason plants grown on such soil have been found 

 to be poisonous to animals including man. Such cases are 

 very rare. 



Turgor pressure is the force of the cell contents against 

 the cell wall. This pressure is produced by the osmotic 

 action of the water through the cytoplasmic membranes. 

 The phenomenon of turgor pressure is important in many 

 activities of the plant. It, as has already been explained, is 

 the force which pushes the root through the soil (see page 

 36) . It is the pressure which causes the stretching of the cell 

 walls in growth. It keeps the leaf and stem cells from wilt- 

 ing. It causes the seed and the pollen grain to germinate. 

 Turgor pressure might be said to be a force in every living 

 cell. 



Turgor pressure acts as a balance on the absorption of 

 water by a plant. Since osmosis is the force by which the 

 water tends to enter the cell, turgor pressure within the cell 

 tends to balance or prevent the entrance of water. When a 

 plant is wilted, it has no turgor pressure, hence there is no 

 force to reduce the entrance of water. When a plant is turgid 

 or has a high turgor pressure, more force must be overcome 

 when water enters. Turgor pressure may vary from zero to 

 several atmospheres, depending on the water and the soluble 

 matter in the cells of the plant and in the soil. 



The turgor pressure of the cells varies through the day. 

 The amount of water tends to decrease because transpiration 

 is faster than absorption. The amount of sugar tends to 

 increase in some plants during the day; in other plants the 

 sugar of photosynthesis is changed to starch with no effect 

 on the cell-sap solution. As the water of a cell decreases 

 and/or the soluble substances increase, the turgor pressure 

 of the cell increases. 



A few mineral salts are being found which do not obey 

 the old commonly accepted laws of diffusion in their absorp- 

 tion by plants. Potassium, for example, is absorbed in much 



